[leebca]

A More Technical Discussion of Fish Stress

Intro

I don’t prefer to write in technical or scientific terms. My personal belief is that, anything (like terminology) that separates the aquarist from useful information is of little value in this hobby. For that reason (and others) I write (hopefully) in a way to make the message both meaningful and easy to understand. That was the foundation of this post: Stress (and the Single Marine Fish) where I could provide some practical, non-technical information to the hobbyist. My emphasis in this post will be the physiological effects of stress on marine fishes.

What I don’t want to imply by the above referenced post, is that fish stress is a huge mystery. Fish stress has a technical aspect and much sophisticated information on fish stress has been gathered through study, tests, research, and data collection. I’d like to make the reader aware of some of the results of those efforts.

I didn't make this a sticky, since it may be of minimal value to most hobbyists. But, if there is a curiosity about this subject, this post may serve to whet the appetite.

As usual, this information comes to the hobby through work performed with and on food fishes. Most of that work is done on freshwater fishes. Some have been done on marine food fishes, but not much. How much of this information applies to ornamental reef fishes is debatable, but I believe, in general, there is some degree of applicability, even if it isn’t 100%. So, let’s get going!


Definition

This is one area I like simplicity, but the high-tech people obtaining research monies have to appear more scientific. So in more technical terms, fish stress is:
A factor or situation which induces physiological changes, which can impact energy use, osmoregulation, respiration, and immunity, including situations which fall outside the fish’s normal realm of tolerance.

That’s a mouthful. In my semi-meaningful yet simplistic terminology, stress is what causes the fish to be uncomfortable. “. . .induces physiological changes. . .” = changes the internal chemical functions and/or internal chemistry of the fish. “. . .outside the fish’s normal realm of tolerance. . .” = bad water quality and/or bad water parameters (e.g., pH) and/or tank mate issues, etc. which in the wild the fish wouldn't/doesn't face. The list of ‘environmental’ stressors is extensive and are comprehensibly covered (for the most part) in the other post: Stress (and the Single Marine Fish)


Stress Effects

The definition sort of says what can be the expected results of stress. But in more precise terms, some outward results of stress on a fish include:
no visual signs
abnormal behavior
increase in breathing
increase in blood pressure
decrease in immune response
decreased reproductive performance
increased susceptibility to disease, and
death

It’s of no surprise that the home hobbyist won’t be taking their fish’s blood pressure. Increase in breathing is caused by dozens of different reasons, so that doesn’t help much. Noteworthy is that 'normal' breathing rate is between 80 and 90 per minute. Abnormal behavior is elusive, unless the aquarist has extensive experience with that species of fish.

If the reader can imagine stress in fish being similar to people with diabetes, it might become more clear on what effects to expect. In a room full of people can the diabetics be identified? No. But if clinical test results were available and the normal and abnormal internal chemistries were known, there would be more information to determine the diabetics from those who aren't. In some respects, a fish suffering from stress may not have an outward appearance of its condition, but there are clinical/internal chemical signals. Read on to the next section and it will become apparent that blood measurements can identify a fish suffering from stress. Those measurements include:
catecholamines
cortisol
corticosteroids
glucose

Lastly, through post mortem exams (necropsy) of internal organs, including reproductive organs, can be examined for signs attributed to unknown stressors after disease is eliminated. The reproductive organs should have developed properly (timely) and when they don't it is often because the fish was the target of some stressor.


Physiological Changes

Things are getting more technical now. If you can’t follow along, don’t be discouraged.

A fish can actually adapt to some minor stressors for a short period of time. But this adaptation is not without a price. Such adaptation depletes energy reserves. In these cases, the fish may appear to be normal to the aquarist, but something is 'falling apart' inside the fish.

Still worth mentioning I think (and relevant to the section on Physiological Changes) is that stress needs to be considered by the aquarist as an ongoing thing. It is not like an injury which happens and now the fish has to heal. It’s a continuous and can be a very subtle thing.

I think most seasoned aquarists (which resist or don’t use a quarantine method) have observed that during a disease outbreak, not all fish get infected, or not all fish get infected to what appears to be, to the same degree or level. That is because each individual fish (not just the species) has differences that dictate its responses to pathogens (monerans (bacteria), protists (one-celled animals and plants), fungus, parasites and viruses). These differences are attributed to genetics, nutrition, handling, injury, care for that particular fish, etc. By “care” in the hobbyist sense, it would be like the fish obtaining different attention than other fishes in the same system.

Part of the above mentioned differentiation is the physiological (internal chemical) response the fish makes to the stressor. Each fish has a varying degree of altered physiology which affects their ability to ward off disease.

One physiological response that has been studied is that fish under stress will release various hormones from the adrenal gland. These hormones decrease the fish’s ability to initiate or sustain an inflammatory response. That is to say that a stressed fish will release hormones that suppress the immune system. This leads to more susceptibility to a pathogen, since the inflammatory response is a key factor in the process to fight disease. [Francis-Floyd, R. CIR919, Fisheries and Aquatic Sciences Dept., FL Cooperative Extension Service, Inst. of Food and Agricultural Sciences, Univ of FL, July, 2002]

What exactly is a treatment? Most fish pathogen disease treatments are really and should be though of as this: A pathogen treatment weakens the pathogen enough so that the fish’s own defenses can overcome the pathogen.

The reader should be able to see then, how stress leads to reduced resistance to disease, and how continued stress can reduce the fish’s ability to recover from a disease. This is a reason to question stress before, during, and after disease treatments. If the stressors aren’t removed or significantly reduced, treatments may either not be effective, or the fish will return to its state of susceptibility to the pathogens. This could render a treatment semi-useless.

Another aspect of the hormone release and adrenal gland activity. . .Fish store simple carbohydrates (like sugars) for emergency use (like putting distance between them and a barracuda!). The hormone released by the adrenal gland releases the stored carbohydrates to make them available to the stressed fish. Since stress is considered continual (see above) the fish comes to the end of its reserves and now has no stored energy to fall back on in a fight with a pathogen, nor to continue to adapt to the stressor(s). In short, things quickly fall apart.

In the absence of available energy (the stored carbohydrates) the fish stops growing. I guess this doesn’t seem like much unless the reader understands that throughout the entire life of a fish, the fish continues to grow. In our aquariums, the fish size may be stunted, but the fish continues to grow. If the fish is not obtaining needed nutrients, under stress, and/or in a poor environment, the fish stops growing. So aquarists who boast, “My fish never got above 5 inches in length” are saying in effect, ‘My fish has been stressed, undernourished, and/or kept in a tank affording a poor environment, so my fish stopped growing.’

Osmoregulation is affected by stressors in the sense of how the fish will regulate mineral metabolism. Simply put (I hope) osmoregulation is the ability of a salt water fish to prevent it from dehydrating from water loss to its environment. Many organs are involved in this process. [Greenwell MG, Sherrill J, Clayton LA. 203. Osmoregulation in fish. Mechanisms and clinical implications. Vet Clin North Am Exot Admin Pract. 6(1):169-89]. When the fish is short on energy, the amount of energy for osmoregulation to continue properly increases making it harder for the fish to perform this vital function. It's a bit of a 'catch.' The less available the energy, the more energy is needed to do the same job.

One particularly interesting point should be mentioned (even though this section of the post could go on and on and on. . .). The effect of temperature changes. Humans don’t relate to the sensitivity a fish has to a temperature change (up or down). But it is known that a small temperature change has an impact on the fish’s ability to ward off a disease.

Most seasoned aquarists (mostly those that resist or don’t us a quarantine process) may see a disease outbreak in their marine aquarium right after a ‘rapid’ temperature change. A fish needs time to acclimate to water chemistry and parameter changes. See It Was Acclimation, I know. . .

In the case of temperature shifts, what happens (on a more technical level) is that the fish’s system doesn’t acclimate fast enough to the change. This produces in the fish a slowed immune response. Should there be any pathogens around, they now have the opportunity to take advantage of the now disadvantaged fish with the slow immune response. The fish’s immune response is a mechanism for the fish to produce antibodies to invading pathogens. If this response is slowed or compromised by a temperature shift, the immune response delay gives the pathogen the advantage. What the aquarist sees after the temperature shift, is a disease outbreak in part due to this slowed response. [Francis-Floyd, R - same reference given above - ]


Prevention and Summary

I’ve left prevention in more practical terms to the previous post: Stress (and the Single Marine Fish)

Like written in the Intro: Stress prevention is continuous. Like stress is ongoing and continuing, so must prevention be ongoing and continuous. The aquarist must always be vigilant to stress (from the fish's point of view).

The reader sees from the above some of the more technical aspects of fish stress. I hope this provides a broader picture of stress from a fish physiological perspective. I tried to soften the technical aspects without getting bogged down in too many new words or phrases. Hopefully, this will be of some interest.